In a heat-sensitive recording process, leuco type heat-sensitive recording materials have generally been employed. However, these heat-sensitive recording materials have the disadvantage that color development occurs in unexpected areas due to severe handling or heating after recording or adhesion of solvents and the like, resulting in stains of recorded images. Extensive studies have recently been conducted on diazo type heat-sensitive recording materials free from the above-described disadvantage.
For example, Japanese Patent Application (OPI) No. 123086/82 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), Gazo Denshi Gakkai-Shi, 11, 290 (1982), disclose diazo type heat-sensitive recording materials containing a diazo compound (diazonium salt), a coupling component and a basic component (inclusive of substances capable of becoming basic upon heating) which are heat-recorded and then irradiated with light to decompose the unreacted diazo compound to thereby stop color formation. According to this process, color formation in unnecessary areas can be effectively prevented; that is, the developed color can be fixed. Nevertheless, these recording materials still exhibit, in some cases, unfavorable coloration (fog) through the gradual progress of pre-coupling during preservation. Therefore, it has been proposed to avoid contact of color forming components by incorporating any one of the color forming components in the form of discontinuous particles, i.e., as a solid dispersion, to thereby prevent pre-coupling. However, preservability of such recording materials before recording (hereinafter referred to as working preservability) is still unsatisfactory and, moreover, there arises the additional disadvantage of reduction in heat developability.
Another proposal to minimize contact between the diazo compound and the coupling component comprises providing these components in independent layers as disclosed, e.g., in Japanese Patent Application (OPI) No. 123086/82. Although this process succeeds in achieving satisfactory improvement of working preservability, the recording materials are impractical due to excessive reduction of heat developability for use in high speed recording having a short pulse width.
In order to satisfy both of working preservability and heat developability, it is also known to separate the coupling component and the basic substance by encapsulizing either one of the two components using non-polar waxy substances (as disclosed in Japanese Patent Application (OPI) Nos. 44141/82 and 142636/82) or hydrophobic high polymers (as disclosed in Japanese Patent Application (OPI) No. 192944/82). In these methods, since the formation of capsules is carried out by dissolving or dispersing the color forming component in a solution of the wax or high polymer in a solvent therefor, the capsules obtained are different from common capsules comprising a core and a shell. When the color forming component is dissolved in the wax or high polymer solution, it is uniformly mixed with the encapsulizing material without forming a core. As a result, pre-coupling gradually proceeds on the interface of capsule walls during preservation so that working preservability cannot be ensured. On the other hand, when the color forming component is dispersed for encapsulization, the color formation reaction does not take place unless the capsule wall is heat-melted, so that heat developability is decreased. Also, the solvent used for dissolving the wax or high polymer should be removed after the formation of capsules.
In order to overcome these problems, the inventors previously developed a process for producing microcapsules comprising a core material containing at least one of the reactants for color formation and capsule walls formed by polymerization to provide excellent heat-sensitive recording materials as disclosed in Japanese Patent Application (OPI) No. 190886/84 (corresponding to U.S. Patent Application Ser. No. 600,267 now abandoned).
However, when the above-described encapsulization process is applied to diazo type heat-sensitive recording materials in which a black color is formed by reacting an appropriate combination of a diazo compound and a coupler, reduction of preservability is sometimes noted and also it is not always easy to stably develop a black color. In particular, a considerable difficulty is encountered in securing stable black color against variation of printing energy of a heat-sensitive printer.